Nowadays, a high percentage of the population in modern society suffers from various kinds of sleep disturbances, due to the impact of increased physiological and socio-cultural stress factors. One of the most common sleep complaints, which the human being has suffered from since ancient times, is insomnia. Although its literal meaning is a total lack of sleep, in clinical and practical terms insomnia has come to refer to a difficulty in initiating and/or maintaining sleep, or non-restorative, non-refreshing sleep. In a survey conducted in the general population of different countries, the prevalence of insomnia was estimated to be about one third of the adult population. The study of sleep disorders in the general population involves several methodological issues that need to be defined prior to proceeding.